span value_type chunk size_t at const noexcept if storage_ index size_

span<value_type> chunk(size_t at) const noexcept {
            if (storage_.index() == 0) {
                size_t offset = at * chunk_size;
                return { std::get<0>(storage_)[at].data() + offset, std::min(std::get<0>(storage_)[at].size() - offset, chunk_size)};
            }
            return std::get<1>(storage_)[at];
        }